1. Field of the Invention
The present invention relates to a process for forming bundles of high modulus polyethylene fibers, and more particularly, to bundles of such fibers which can be used as reinforcing agents for brittle matrices.
2. Description of the Prior Art
The use of various types of fibers for the reinforcement of brittle matrices is well-known in the prior art. For example, a paper entitled "Fibrous Reinforcement For Portland Cement" by S. Goldfein, "Modern Plastics" (April 1965), pages 156-159, discloses that the impact resistance and flexural strength of castings and moldings made from Portland cement mixtures can be improved by the addition of organic fibers such as nylon, polypropylene, and polyethylene fibers. As noted by Goldfein, the high pH of the cement slurry prevents the use of glass, cotton, rayon, acetate and Dacron (trademark of E. I. du Pont de Nemours & Co. for a polyester fiber made from polyethylene terephthalate). As noted by Goldfein, a limiting factor was the viscosity effect of the fibers on the cement. Nylon was limited to a maximum of about 3% whereas the other fibers could be used at quantities as high as 7%.
The difficulties in using such fibers as reinforcement are described in part in U.S. Pat. No. 3,591,395 to Zonsveld et al. As noted therein, the use of 3% by weight of nylon or 6% by weight of polypropylene would be economically prohibitive. Furthermore, the use of low denier monofilaments chopped into short lengths entails some technical difficulties. Such filaments are normally marketed wound on small diameter spools. A deformation of such filaments is thus generated which, after winding off, manifests itself as a tendency to curl. The resulting monofilaments of short lengths are therefore difficult to handle, since the fibers ball together and cannot be distributed evenly in a water-hardenable mass. Unraveling of the fibers, as mentioned by Goldfein in his paper, is a cumbersome and time-consuming operation which is commercially unacceptable.
In an attempt to solve this problem, Zonsveld et al advocate the use of 0.05 to 2% by weight of fibrous reinforcing elements formed from a stretched and then fibrillated plastic film material which is preferably polyolefin film. The Zonsveld et al patent contemplates the use of either continuous filaments or short segments of fibrillated plastic film material.
The same problem of the tendency of fibers to ball up in concrete products rather than being mixed uniformly is mentioned in Goldfein, U.S. Pat. No. 3,645,961. As stated therein, it is preferred that longer fibers be used since the strength increases with the length of the fibers, but that the difficulty of mixing increases at the same time. That is, as the length of the fibers is increased, there is a greater tendency of the mixture to ball.
In fact, this problem with the balling of fibers is sometimes employed to advantage to produce low density concretes which are referred to as "air-entrained" concretes and have an air content of up to about 10%. Thus, U.S. Pat. No. 3,679,445 to Howe describes the manufacture of such concretes, and suggests the use of fibers which are less than two inches long and usually range in length between about 1/8 inch and 1 and 1/8 inches. Included within the suggested fibers are those made from cotton, flax, rayon, Orlon (trademark of E. I. du Pont de Nemours & Co. for an acrylic fiber), nylon, Dacron (trademark of E. I. du Pont de Nemours & Co. for a polyester fiber made from polyethylene terephthalate), Terylene (trademark of Millhaven Fibers Limited for a polyester fiber based on terephthalic acid), polyethylene, polypropylene, polyvinyl chloride and glass. Although there are certain applications wherein such air-entrained concretes are desirable, such is not the case where structural support is needed.
As noted in Oya et al, U.S. Pat. No. 3,865,779, the hydrophobic properties of synthetic fibers when they are admixed with mortar, result in their being poorly dispersed in the mortar and being liable to float on the surface, which inevitably results in the product having nonuniform physical properties. Accordingly, Oya et al discloses a process for preparing reinforcing additives to be applied to inorganic cements which are prepared by combining certain polymers, inorganic materials or cement, and a surfactant; mixing and melting the mixture; and then extruding it into fibers of desired shapes. However, the extra steps involved in such a process severely limit its commercial viability.
Another method of mixing fiber reinforced concrete without the formation of fiber balls therein is disclosed in Dearlove et al, U.S. Pat. No. 4,823,706. The method includes the steps of depositing a uniform layer of substantially individual concrete reinforcing fibers on an elongated web, coiling the web to contain the fibers, locating the coiled web in proximity to a concrete mixing device, and progressively unrolling the web at a predetermined rate to discharge the layer of fibers therefrom into the mixture. The invention described therein is stated to be an alternative to specially designed fiber feeders which separate fiber balls found in a package of fibers and slowly feed individual fibers into a mixer or the like. However, both of these solutions entail the use of extra steps and equipment which have a deleterious effect upon the commercial value of the processes.
A number of patents disclose the use of fibers in various forms as reinforcement in composites which include brittle matrices. For example, Fischer et al, U.S. Pat. No. 3,533,203, describes the use of highly elastic multifilament polypropylene strands, twisted or untwisted, i.e., ropes or rovings, as the pretensioning element for precompressed concrete. After tensioning and holding such strands by clamps, concrete is poured around the tensioning strands and allowed to set while maintaining the strands under tension.
Suzukawa, U.S. Pat. No. 3,607,685, discloses a composite building laminant containing an inorganic cement reinforced with polypropylene multifilaments.
Reineman, U.S. Pat. No. 3,922,413, and Duff, U.S. Pat. No. 3,949,144, both disclose a concrete structural member comprising a plurality of alternate layers of epoxy resin containing concrete and fiber reinforced epoxy resin. The fiber reinforcement may be synthetic fibers woven in a basket-like weave. It should be noted that the fiber reinforcement is used in the epoxy resin layers and not in the concrete.
Downing et al, U.S. Pat. No. 4,240,480, discloses cementitious composites that may include synthetic organic polymer fibers, such as polypropylene fibers, in various forms, for example roving or fibrillated sheet, as reinforcement. The compositions are prepared by subjecting a mixture of hydraulic cement, fine aggregate, a selected water dispersable polymer and water to a homogenization process in order to substantially reduce voidage in the product, and then curing and drying the product. It should also be noted that in each of the examples therein, the ingredients are mixed in a mixer and extruded before being cured and dried. Such a process would not be suitable for forming large batches of material.
Accordingly, a need exists for fibers in a form in which they can be used as reinforcements in composites, wherein the fibers are readily mixed with a brittle matrix without the tendency to balling and air entrapment which results in the nonuniformity found in the prior art, or the use of expensive and complicated equipment to avoid such tendencies.